Werner Steuernagel
Vice President — Engineering
Cable Manufacturing & Assembly Co.
Bolivar, Ohio

Edited by Kenneth Korane

Mechanical control cables provide a simple, lightweight, economical, and reliable way to activate throttles, latches, gas springs, electromechanical devices, and many other mechanisms. They're widely used in office furniture, recreational vehicles, lawn mowers, and medical devices, as well as adjustable seats in cars and planes.

Mechanical control cables feature either a solid-wire or wire-cable core housed within a conduit. They provide a simple and reliable method to activate throttles, latches, and other mechanisms.

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The basic design features a movable core — either a solid-wire or braided wire-rope cable — that's free to travel axially inside a conduit. Actuating a lever or similar device at one end of the cable assembly produces output force and motion at the other end.

Solid-core controls are generally used to transmit force in both push and pull directions. The ends of solid wires can be formed to eliminate the need for separate fittings and terminations. But the solid wire requires large bend radii and simple routing to avoid kinks, drag or surface friction, and permanent set. All push-pull controls have greater load capacity in tension than compression — they can pull more than they can push.

In some cases stiff, small-diameter wire-rope cables can be used in push-pull applications, provided push loads are light and the cable and conduit are carefully matched. However, these so-called flexible-core controls are usually found on pull-pull controls that transmit tensile force in both directions. For high-load, push-pull applications, specially designed wire cores are available which maintain high flexibility yet permit loads to 100 lb.

In general, more-flexible conduits and cores provide greater routing freedom and smaller bend radii in restricted installations. They often feature return springs, which maintain specified loads on the cables and return mechanisms to their original position after activation.

DESIGN FACTORS

Engineers should consider parameters such as load, routing, friction, stretch, permanent set, lost motion, temperature, environment, and exposure to contaminants when specifying cables. Here's a look at these critical factors.

Load factors. For push-pull controls, the cable assembly's rated working loads should be in the pull or tension mode. Push, or compression, loads should be ≤50% of pull loads. Reducing the push load minimizes a core's tendency to displace the conduit and, more importantly, reduces the potential for the unsupported core outside the conduit to kink, bend, or distort.

Base maximum pull-pull working loads on the cable's minimum breaking strength, plus a safety factor. Also consider the conduit's resistance to deflection and compressive forces, and cable-assembly end-fitting selection. High loads and cycles can cause the cable to stretch and wear through the conduit liner. Core and conduit must remain as originally routed for an assembly to function properly.